According to the so-called BMA method, lower hydrocarbons, especially methane, are reacted with ammonia at temperatures of approximately 1000.degree. to 1350.degree. C. in the presence of a catalyst to form hydrogen cyanide and hydrogen (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th edition (1987), vol. A8, pp. 162-163). The strongly endothermic reaction normally takes place in tube reactors. The reaction tubes consist essentially of aluminum oxide. They are suspended in a BMA tube reactor and are heated externally. The inner surfaces of the tubes are provided with a catalytically active coating, usually containing platinum.
According to the method of German Patent Application DE-AS 10 13 636, the reaction gases are conducted over catalysts which contain, in addition to one or more platinum metals (especially platinum), aluminum or magnesium in metallic form or as a nitride. A preferred, catalytically active coating of carriers of the shaped articles, which consist essentially of aluminum oxide, contains platinum and aluminum nitride. In order to produce the coating, the shaped articles are impregnated with e.g. aluminum-containing solutions of chloroplatinic acid; after drying, a reduction is carried out at 600.degree.-900.degree. C. with hydrogen; the formation of the nitride takes place in the BMA reactor in the start-up phase, under the influence of the ammonia used for the HCN synthesis. In order to obtain a catalytically active layer which is sufficiently thick for continuous duty--approximately 10 to 30 mg platinum per cm.sup.2 surface--, the impregnating and hydrogenating step must generally be repeated several times, as is emphasized in U.S. Pat. No. 3,112,215, because only approximately 2 mg platinum per cm.sup.2 coating can be deposited per impregnation process. Thus, the production of the catalytically active coating by this method requires using a large amount of precious metal and high expense.
The previous method was improved in accordance with the disclosure of German Patent Application DE-OS 37 23 535 as regards the use of noble metal. It was found that a catalytic solution having an atomic ratio of noble metal, especially platinum, to aluminum in a range of 0.001-0.1: 1 permits the production of coatings with good catalytic action. However, after the impregnation with a solution containing e.g. metal halides, and after drying, reduction with hydrogen was required. In addition, it proved to be advantageous to carry out the impregnation and the hydrogenation several times in order to obtain coatings having long service life and a high level of performance.
The application of metals from the vapor phase is also known from German Patent DE 10 13 636. In this case, readily decomposable or readily reducible compounds of platinum and aluminum are applied in the presence of reducing gases, e.g. in a flame, while an aluminum-containing platinum coating is precipitated onto the shaped article. This method was not accepted in practice because the production of volatile noble-metal compounds is expensive and the practical problems of depositing a coating on the inner surface of long tubes could not be solved in a satisfactory manner. In addition, multiple coatings had to be applied.
A further problem of the previously known coating method using soluble or vaporizable compounds of platinum metals arises because of the risk of allergic reactions of sensitive workers performing the coating. This risk required special precautions so that the coating method could be carried out in such a manner that industrial hygiene problems are avoided or at least reduced.